The grafting of synthetic polymers onto a cellulosic backbone has been the subject of research activities for a long time with the object of producing a polymer that has the beneficial properties of both cellulose and the synthetic polymers. Enormous research and development efforts have occurred over the last 40 years, but no polymer or process has yet been discovered which has proceeded to commercialisation.
The grafting of polymers on a cellulosic backbone proceeds through radical polymerisation wherein an ethylenic monomer is contacted with a soluble or insoluble cellulosic material together with a free radical initiator. The radical thus formed reacts on the cellulosic backbone (usually by proton abstraction), creates radicals on the cellulosic chain, which subsequently react with monomers to form graft chains on the cellulosic backbone. Related techniques use other sources of radical such as high energy irradiation or oxidising agents such as cerium salt or redox systems such as thiocarbonate-potassium bromate. These methods are well known, see, eg, McDonald, et al. Prog. Polym. Sci. 1984, 10, 1; Hebeish et al, “The Chemistry and Technology of cellulosic copolymers”, (Springer Verlag, 1981); Samal et al. J Macromol. Sci-Rev. Macromol. Chem. 1986, 26, 81; Waly et al, Polymers & polymer composites 4,1,53,1996; and D. Klenn et al, Comprehensive Cellulose Chemistry, vol. 2 “Functionalization of Cellulose” pp. 17–31 (Wiley-VCH, Weinheim, 1998).
Another strategy involves functionalising the cellulose backbone with a reactive double bond and polymerising in the presence of monomers under conventional free radical polymerisation conditions, see, eg, U.S. Pat. No. 4,758,645 (Nippon Paint). Alternatively, a free radical initiator is covalently linked to the polysaccharide backbone to generate a radical from the backbone to initiate polymerisation and form graft copolymers. For example, in U.S. Pat. No. 4,206,108 (Du Pont), a thiol is covalently bound to a polymeric backbone with pendant hydroxy groups via a urethane linkage; this polymer containing mercapto group is then reacted with ethylenically unsaturated monomers to form the graft copolymer.
Unfortunately, none of these techniques lead to a well-defined material with a controlled macrostructure and microstructure. For instance, none of these techniques leads to a good control of both the number of graft chains per cellulose backbone molecule and molecular weight of the graft chains. Moreover, side reactions are difficult, if not impossible, to avoid, including the formation of un-grafted polymer, graft chain degradation and/or crosslinking of the grafted chains.
In an attempt to solve these problems, pre-formed chains have been chemically grafted onto cellulosic polymers. For instance, in U.S. Pat. No. 4,891,404, polystyrene chains were grown in an anionic polymerisation and capped with, eg, CO2. These grafts were then attached to mesylated or tosylated cellulose triacetate by nucleophilic displacement. This method is difficult to commercialise because of the stringent conditions required by the method. Moreover, the set of monomers that can be used in this method is restricted to non-polar olefins, thus precluding any application in water media.
Block copolymers based on cellulose esters have been reported. See, eg, Oliveira et al, Polymer, 35, 9, 1994; Feger et al, Polymer Bulletin, 3,407, 1980; Feger et al, Ibid, 6, 321, 1982; U.S. Pat. No. 3,386,932; Steinmann, Polym. Preprint, Am. Chem. Soc. Div. Polym. Chem. 1970, 11, 285; Kim et al, J. Polym. Sci. Polym, Lett. Ed., 1973, 11, 731; and Kim et al. J Macromol. Sci., Chem (A) 1976, 10, 671. A major problem with these references is the generation of considerable chain branching, grafting or crosslinking. Mezger et al, Angew. Makromol Chem., 116,13,1983 prepared oligomeric, monohydroxy-terminated cellulose coupled with 4,-41-diphenyldisocyanate, which was then used as a UV-macro-photo-initiator to prepare triblock copolymers. The reaction is known as the iniferter technique and uses UV initiation, which limits its applicability to certain processing methods. Furthermore, it is typically applicable to styrenic and methacrylic monomers. Other monomers, such as acrylics, vinyl acetate, acrylamide type monomers, which are in widespread use in waterborne systems, might require another technique.
So-called “living” radical polymerisation techniques are known which can give better defined polymers in terms of molecular structure. Three approaches to preparation of controlled polymers in a “living” radical process have been described (Greszta et al, Macromolecules, 27, 638 (1994). The first approach involves the situation where growing radicals react reversibly with scavenging radicals to form covalent species. The second approach involves the situation where growing radicals react reversibly with covalent species to produce persistent radicals. The third approach involves the situation where reaction which regenerates the same type of radicals. However, none of these techniques have been successfully applied to polysaccharide substrates.
It has previously been recognised in the art that cellulose based materials adhere to cotton fibres. For example, WO 00/18861A and WO 00/18862A (Unilever) disclose cellulosic compounds having a benefit agent attached, so that the benefit agent will be attached to the fibre. See also WO 99/14925A (Procter & Gamble). However, the ability of polysaccharide, especially cellulose based materials to adhere has not been fully investigated, and a need exists to find polysaccharide based materials that are of commercial significance.
There is therefore a strong need to develop a process that makes it possible to prepare grafted materials from polysaccharide polymers, with a predictable number of graft chains per polysaccharide backbone. These graft chains should be controlled in length and chemical composition. Moreover, the method of synthesis should be capable of commercial application. In addition, a need exists to provide polysaccharide-based polymers which provide benefits to fibres and surfaces.